has signed a far-reaching teacher merit-pay bill that will overhaul how teachers across the state will be evaluated and paid.

The law creates an evaluation system that relies heavily on student test score data to judge teacher quality. For new teachers, it also creates a performance-based pay system and ends tenure-like job protections.

Florida's merit-pay push is part of a national effort to improve education by tying teachers' pay to their overall effectiveness.

"We are absolutely changing this country," Scott said during the signing ceremony Thursday at a charter school in

Jacksonville

that aims to boost academic performance among low-income students. He was flanked by students as he put his name on the controversial measure.

Advocates say the law will help Florida schools identify top teachers, reward them financially and assign them to work with their neediest students.

But many teachers along with their statewide union, the Florida Education Assn., are opposed. They say the law will be expensive, will rely on an unproven system and won't fairly evaluate teacher performance. The union has threatened to sue, arguing the plan tramples on teachers' rights to collective bargaining on salaries and work conditions, among other issues.

It was quickly praised as "breakthrough legislation" and a "model of bold reform" by the foundations run by education reformer

Michelle Rhee

and former Gov.

Jeb Bush

, respectively.

But the

American Federation of Teachers

, a national teachers union, said it "took a wrecking ball to the dreams" of Florida's public school students.

The merit-pay bill was pushed by state education leaders and Florida's Republican leadership.

The law will have the most impact on teachers hired after July. Teachers already on the job can retain their current contracts and be paid based on current pay plans — which largely use seniority and advanced degrees to set salaries. But all teachers will be judged by the new test-based evaluation system and can lose their jobs after several years of poor performance.

The state plans to develop a "value-added" system to judge teacher quality with test-score data but take into account factors outside a teacher's control, such as a student's absentee rate.